I found this, started out as good news
http://www.sevenforums.com/sound-audio/92783-there-quick-toggle-loudness-equalization.html
You'll find the registry entry located at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\MMDevices\Audio\Render{guid-of-your-output-device}\FxProperties, just change on/off settings and refresh regedit to identify which one is reflecting what you want to check/un-check.
But after attempting the changes, as noted, even when checking "immediate mode" the registry change does not immediately update the setting, requiring a log off to kick in. I've since hunted for a way to somehow refresh the registry, maybe make a inf file that would be installed ? I'm not sure on that front.
I found this close to the answer, it's all bad news, they say from a software developer point of view, that you just can't do it without writing your own system effect.
http://social.msdn.microsoft.com/Forums/en/windowspro-audiodevelopment/thread/b028b430-48c5-434b-a644-da192fbf20a5